Sad (triste) is a temporary condition, so I would use estar, but what if I am talking about a book's ending? Would I use ser since the ending doesn't change?

'Sad' can be a characteristic of a person (normally sad) or a condition/state (not always so). For the former, you'd use 'ser triste'; for the latter, 'estar triste'. The original post in the thread doesn't mention 'temporary'. Think more in terms of 'characteristic' or 'state'.
The story written in a book can be 'sad', 'uplifting', 'scary', etc. All are used to describe or define the story at a particular moment. Since these are definitive labels, the verb to use is 'ser'. 'Es triste la parte cuando muere uno de los hijos.'

When we talk about a happy ending or a sad ending in a book, it's common to use 'tener un final feliz' or 'tener un final triste'. But you may also say, "El final del libro es triste." That's a defining characteristic.